The focus of the patent is on oilfield applications that include lost circulation, cement repair, sand consolidation and conformance applications. The term conformance describes the management and alteration of water and gas flows in a subterranean environment to optimize hydrocarbon production. The term “water shut-off” is often used interchangeably with conformance. Water shut-off represents a major subset of conformance and refers to such problems as water flow through fractures, thief zones, high permeability streaks and water coning, or lack of integrity in cement. This technology is easily adapted to the grouting and mining industry where there are similar challenges with water shut-off and stabilization. These applications and industries have all used soluble alkali silicates mainly in the form of sodium silicate.
Alkali silicates have an extremely versatile chemistry that allows them to be engineered to solve a great range of conformance and stabilization problems. Depending on choice of setting agents, set times for silicates can range from instant to days. The set silicate can range from a ringing silica gel to a cementitious material. Sodium silicate has been used since the 1920's to solve a wide range of downhole issues in oilfield. The chemistry of sodium silicate for conformance has been well documented in the literature. P. H. Krumrine and S. D. Boyce, Profile Modification and Water Control with Silica Gel-Based Systems, SPE 13578 presented at the 1985 SPE International Symposium on Oilfield and Geothermal Chemistry, Phoenix, Ariz., Apr. 9-11, 1985 is a leading article on the subject. This paper presents the chemistry, properties, benefits, limitations, methods and provides an extensive list of potential setting agents, and is incorporated herein by reference.
Sodium silicate is typically formulated to undergo a gelation reaction or a precipitation reaction. If downhole conditions do not provide a suitable environment for the gelation and/or precipitation of sodium silicate, a setting agent can be used to initiate the reaction. Precipitation systems are formulated to take advantage of sodium silicate reacting with metal cations (e.g., Ca2+, Mg2+, Zn2+, Al3+, Fe3+). The best example of this type of formulation is a solution of calcium chloride placed in the troubled area, followed by a water spacer followed by a solution of sodium silicate. Once mixed, the sodium silicate reacts with the calcium to form calcium silicate.
Given that the reaction between sodium silicate and metal cations is nearly instantaneous, the catalyst cannot be mixed on the surface with the sodium silicate. Rather, the catalyst must be applied separately from the sodium silicate. These systems are typically used near-wellbore.
Gelation systems are formulated with a sufficient quantity of weakly basic, neutral or acidic material to reduce the pH of a sodium silicate solution to a point where the sodium silicate will self-polymerize into a silica gel. It is understood in the art that a great variety of inorganic, organic and natural compounds can initiate this reaction.
Sodium silicate can also be used as a component of a cement mix. Cements can be traditional Portland-based cements as well as Pozzolanic-based materials such as flyash, or blast furnace slag. It is generally accepted that for Pozzolanic-based cements, alkali silicates (sodium silicate) is an effective activator since it acts as a source of alkali and reactive silica.
While sodium silicates are formulated to obtain a specific type of chemical reaction, once placed in a downhole environment several different types of chemical reactions can occur over time. These include cation exchange with mineral surroundings and dehydration.
There are several reasons often cited for choosing sodium silicate-based technology for conformance applications. These reasons include:                initial low viscosity which promotes penetration;        small molecular weight which promotes penetration;        excellent thermal stability;        excellent chemical stability;        high strength on setting;        can be formulated with a wide range of set times;        environmentally friendly; and        cost effectiveness.        
Alkali silicate-based technology does have limitations. Set times can be difficult to control due to exposure to contaminants such as salt, metals and organics. Changes in reservoir conditions and temperature also impact set times. Depending on the contamination, there can also be severe loss of strength. Gelled alkali silicates also have the further disadvantage of syneresis. While alkali silicates have excellent strength and chemical and thermal resistance, there is always a need for higher levels of each. Increased mechanical strength would allow greater use in high temperature and high temperature reservoirs or casing repair applications. Over the years, many processes have been proposed for improving silicate-based technology for blocking, sealing or consolidating high permeability zones, channels, fissures and the like. These methods have been disclosed in U.S. Pat. Nos. 1,421,706; 3,658,131; 4,031,958; 4,257,813; 6,059,036; and 7,740,068. All these patents share the feature of using standard, commercially available sodium silicate as the source of alkali silicate.
Silicates which are used for conformance applications are water soluble silicates which have a weight ratio of SiO2 to Me2O where Me is the alkali metal and is most commonly sodium or potassium. For commercially available aqueous sodium silicate, the weight ratio of SiO2 to Na2O is generally 1.6 to 3.25. For commercially available aqueous potassium silicate, the weight ratio of SiO2 to K2O is generally 1.6 to 2.5. Table 1 below, which is derived from U.S. Pat. No. 7,740,068 to Ballard, presents the composition and typical properties of commercial grades of liquid sodium silicate and potassium silicate.
TABLE 1PQ CorporationSiO2/%%DensityProduct NameMe2O% SiO2Me2OSolids(20° C.)Potassium SilicatesKASIL ® 12.520.88.329.110.5ppgKASIL ® 62.126.512.6539.1511.5ppgKASIL ® 332.124.411.636.011.2ppgKASOLV ® 161.652.832.585.343.0lb/ft3KASIL ® 16241.6515.09.124.110.16ppgKASIL ® 21302.120.09.529.510.6ppgKASIL ® 21352.1824.011.035.011.15ppgKASIL ® 2.52.571.028.499.477.5lb/ft3KASIL ® SS2.571.028.499.457.4lb/ft3AGSIL ™ 25H2.560.6524.2584.9—Sodium SilicatesA ® 16471.628.818.046.813.40ppgA ® 18471.830.2016.7846.9813.24ppgA ® 24452.432.213.445.612.8ppgA ® 24472.4033.213.947.113.0ppgA ® 26452.5832.112.544.612.63ppgBJ ™ 1201.8023.713.1536.8511.9ppgBW ™ 501.6026.216.7542.5512.7ppgC ™ 502.036.0018.0054.014.1ppgD ™2.0029.414.744.112.8ppgE ™3.2227.78.636.311.5ppgK ®2.8831.711.042.712.3ppgM ®2.5832.112.444.512.6ppgN ®3.2228.78.937.611.6ppgN ® 383.2228.78.934.611.3ppgN ® 38 Clear3.2228.78.937.611.6ppgO ®3.2229.59.138.611.8ppgOW ®3.2229.469.1538.61—RU ™2.4033.013.947.113.0ppgSS ®3.2275.723.599.2(11.8)ppgSS ® 223.2275.723.599.21.44g/cm3SS ® 752.7572.926.599.4(11.8)ppgStar ™2.5026.510.637.111.7ppgStarso ™1.8024.1213.4037.5212.0ppgStixso ™ RR3.2530.09.239.211.8ppgV ™2.5026.510.637.111.7ppg
Although the properties vary among these alkali silicates, they share the common characteristic of being infinitely soluble in water. These soluble silicates are commonly used to block and strengthen permeable zones in subterranean formations. These applications include conformance for oil field, grouting for the construction industry, and water shut-off for mining.
Although commonly used, these alkali silicate reactions may be negatively affected as follows:                reaction rate for gelation is sensitive to the presence of salt (NaCl);        gels may exhibit syneresis when formed in excess water; and        fresh water zones require treatment with a multivalent cation to initiate precipitation.        
There is a need for an agent that exhibits the reaction properties of alkali silicates without any of the effects described above. Any further increase in mechanical strength properties would also allow for greater application.